Modern storage systems provide users with the capability of storing and backing up enormous amounts of data quickly over networks among various local and remote locations. In such systems, an initial copy of disks at a primary storage site is made to disks at a secondary storage site. The storage system is typically configured to automatically copy the entire disk, and configure the disks at the primary and remote storage sites as remote copy pairs. In performing these operations, typically an initial copy of the disk at the primary site is made to a corresponding disk at the secondary site. As operations at the primary site change the data there, that changed data is “mirrored” at the secondary site by transmission from the primary site to the secondary site of the changed data.
It is well know, however, that most customers or users of data processing systems do not use the entire storage capability of the hard disk drives situated at their facility. Thus, each hard disk drive will include information which is desired to be backed up or transferred to the secondary or remote site, as well as large amounts of “null” data. This null data represents the contents of all of the portions of the hard disk which are not being used by the customer at that time. For example, the null data may consist of formatted, but unused portions of the disk.
With the sizes of hard disks continuously increasing, substantial time and network resources are being employed to transfer and store this null data at remote sites when the entire hard disk at a primary site is copied in a remote copy operation to a hard disk at the secondary site. This extensive copying of null data depletes system resources and network bandwidth, and is generally undesirable.
The effect has been alleviated to some extent by data compression techniques. Such data compression techniques can reduce network traffic by compressing the data to be transmitted over the network so that it consumes less bandwidth. Examples of such compression technology in storage systems include the UltraNet Edge Storage Router™ sold by CNT of Minneapolis, Minn., or the 9801SNS Storage Network System™ sold by Inrange Technologies Corp. of Lumberton, N.J. Although those products reduce the load on the network in the sense the fewer bits are transferred, they increase the load on the transmitting and receiving units which must compress and decompress the data, and the decompressed data still includes the null data which is written to the target storage subsystem.
What is needed is a method and apparatus which can avoid transmitting and copying null data, thereby reducing the load on the network, the transmitting system and the receiving system.